local $/;
my $file = <IN>;
close IN;

@array=('file', 'this', 'dog', 'forward');

foreach $searchstring(@array) {
  if ($file=~m/$searchstring/i) {
    push @stringsfound,$searchstring;
  }
}

##</code><code>##

@stringsfound=grep {$file=~/$_/i} @array;